The present invention relates to a clock regeneration circuit for regenerating a clock signal for data demodulation from an AM data multiplex demodulated signal which is obtained by multiplexing a digital modulated signal and an AM modulated signal in the same frequency band and at the same time.
A conventional data multiplex method is roughly classified into a time division multiplex method used by character multiplex broadcasting of television broadcasting and a frequency multiplex method used by character multiplex broadcasting used by FM broadcasting. The frequency band occupied by AM broadcasting is narrow so that frequency multiplex and time division multiplex cannot be used. An AM data multiplex modulated signal generating apparatus for multiplexing an AM modulated signal and a digital modulated signal in the manner similar to data multiplex broadcasting in television or FM broadcasting, has not been realized as yet.
The applicant of the present application has proposed an apparatus for generating an AM data multiplex modulated signal to be obtained by multiplexing an AM modulated signal and a digital modulated signal, the apparatus not adversely affecting an AM synchronous detection output when the AM data multiplex modulated signal is AM-synchronously detected (JP-A-8-166636).
The specific contents of an AM data multiplex modulated signal modulated in the above manner will be later detailed. Since a digital modulated signal and an AM modulated signal are multiplexed in the same frequency band and at the same time, a modulating method is different from that used by time division multiplexing or frequency multiplexing. It is, therefore, impossible to reproduce a clock signal used for signal demodulation through synchronous detection, by extracting a desired digital modulated signal at a selected data multiplex time and frequency band as in conventional cases.
It is an object of the present invention to provide a clock regeneration circuit for regenerating a clock signal used for data demodulation from an AM data multiplex modulated signal obtained by multiplexing an AM modulated signal and a digital modulated signal in the same frequency band and at the same time.
According to one aspect of the present invention, a clock regeneration circuit for regenerating a clock signal for data demodulation from an AM data multiplex modulated signal obtained by multiplexing a digital modulated signal and an AM modulated signal in the same frequency band and at the same time, comprises carrier extracting means for extracting a carrier of the AM modulated signal from the AM data multiplex modulated signal, phase comparator means for receiving as one input the carrier extracted by the carrier extracting means, a voltage-controlled oscillator whose oscillation frequency is controlled by an output from the phase comparator means, and frequency-dividing means for frequency-dividing the oscillation frequency of the voltage-controlled oscillator and supplying a frequency-divided output to the phase comparator means as another input.
In the clock regeneration circuit of this invention, the carrier extracting means extracts the carrier of the AM modulated signal from the AM data multiplex modulated signal, the phases of the extracted carrier and an output of the frequency divider are compared by the phase comparator, and in accordance with a phase comparison result, the oscillation frequency of the voltage-controlled oscillator is controlled. Accordingly, the oscillation output of the voltage-controlled oscillator is synchronized with the carrier of the AM modulated signal, and the oscillation output of the voltage-controlled oscillator can be used as the clock signal for demodulating the digital modulated signal separated from the AM data multiplex modulated signal.
As the frequency divider, a direct digital synthesizer may be used which is input with an oscillation output from the voltage-controlled oscillator and supplies an output to the phase comparator as the other input.
According to another aspect of the invention, a clock regeneration circuit for regenerating a clock signal for data demodulation from an AM data multiplex modulated signal obtained by multiplexing a digital modulated signal and an AM modulated signal in the same frequency band and at the same time, comprises frequency-converting means for frequency-converting the AM data multiplex modulated signal into an intermediate frequency, carrier extracting means for extracting a carrier of the AM modulated signal from the AM data multiplex modulated signal frequency-converted into the intermediate frequency, phase comparator means for receiving as one input the carrier extracted by the carrier extracting means, a voltage-controlled oscillator whose oscillation frequency is controlled by an output from the phase comparator means, and a first direct digital synthesizer for inputting an oscillation output from the voltage-controlled oscillator and supplying an output to the phase comparator means as another input, and a second direct digital synthesizer for inputting the oscillation output from the voltage-controlled oscillator and supplying an output to the frequency-converting means as a local oscillation output, wherein the oscillation output of the voltage-controlled oscillator is used as a clock signal for data demodulation from the AM data multiplex modulated signal.
According to this clock regeneration circuit of this invention, the carrier extracting means extracts the carrier of the AM modulated signal from the frequency-converted AM data multiplex modulated signal, the phases of the extracted carrier and an output of the first direct digital synthesizer are compared by the phase comparator, and in accordance with a phase comparison result, the oscillation frequency of the voltage-controlled oscillator is controlled. The oscillation output of the voltage-controlled oscillator is supplied to the frequency-converting means as a local oscillation output. Accordingly, the oscillation output of the voltage-controlled oscillator is synchronized with the carrier of the AM modulated signal, and the oscillation output of the voltage-controlled oscillator can be used as the clock signal for demodulating the digital modulated signal separated from the AM data multiplex modulated signal.